Conventionally, a lean NOx catalyst has been known in which, when the exhaust is an oxidizing atmosphere, (when the concentration of oxygen in the exhaust is high relative to the concentration of reducing components (HC, CO)), nitrogen oxides (NOx) in the exhaust is absorbed, and when the exhaust is a reducing atmosphere (when the concentration of reducing components in the exhaust is high relative to the concentration of oxygen), the NOx thus absorbed is reduced. In an exhaust purification device provided with such a lean NOx catalyst, the absorption and reduction of NOx is repeated by making the exhaust air-fuel ratio (mass ratio of air to combustible gas in the exhaust path) to be lean and then rich.
As a method for controlling the exhaust air-fuel ratio, there is a method in which the exhaust air-fuel ratio is made low (hereinafter referred to as “enriching”) by reducing the intake air amount of the engine and adjusting the fuel injection (hereinafter referred to as “main injection”) amount contributing to torque (hereinafter referred to as “method by rich combustion”), and a method in which the exhaust air-fuel ratio is enriched by performing fuel injection that does not contribute to torque (hereinafter referred to as “post injection”) to flow unburned fuel into the exhaust path (hereinafter referred to as “method by post rich”). In addition, alternatively, a method has also been known in which fuel is directly injected into the exhaust path (hereinafter referred to as “method by exhaust injection”).
In Patent Document 1 and Patent Document 2, inventions that apply this method by exhaust injection are exemplified.
More specifically, in Patent Document 1, an exhaust purification device is further exemplified in which a lean NOx catalyst is provided in an exhaust path, an oxidation catalyst is disposed upstream of this lean NOx catalyst, and a reducing agent such as diesel oil is supplied from upstream of this oxidation catalyst. In this exhaust purification device in particular, at an initial stage among the intervals in which the reducing agent is supplied, combustion of the reducing agent in the oxidation catalyst is promoted by way of including hydrogen, which is high in low temperature oxidizability, in the reducing agent, and the exhaust is made a reducing atmosphere even in a case where the exhaust temperature is low, while reduction of NOx in the lean NOx catalyst is promoted.
In addition, in Patent Document 2, an exhaust purification device is exemplified that has an oxidation catalyst in the exhaust path of a diesel engine, and supplies a reducing agent such as diesel oil to this oxidation catalyst. In this exhaust purification device in particular, the intake air flow rate into the diesel engine is no more than a predetermined value, and in a case where the temperatures of the oxidation catalyst and exhaust are at least a predetermined temperature at which NO is oxidized by the oxidation catalyst to generate NO2, the reducing agent is supplied, whereby emission of NO2 is controlled.
However, in the methods by exhaust injection such as those exemplified in Patent Documents 1 and 2, and the method by post rich described above, when a reducing agent is supplied in a state in which the exhaust temperature is low, the exhaust gas temperature declines further due to the latent heat of vaporization of the reducing agent, and the reducing agent may remain in the exhaust path without vaporizing. In addition, in a case of, immediately after startup of the engine, there being a state in which the lean NOx catalyst has not achieved the activation temperature, reduction of NOx cannot be promoted, even if the exhaust air-fuel ratio is made low by supplying such a reducing agent.
Moreover, when enriching the exhaust air-fuel ratio by a method of rich combustion as described above, the operating conditions are limited. For example, during high-load operation in which combustion is sharp, the combustion noise grows worse. In addition, during low-load operation such as immediately after startup of the engine or while idling, the charge efficiency into the cylinders decreases and combustion may become unstable.
Accordingly, an exhaust purification device has been proposed that reforms fuel with a reforming catalyst to produce a reducing gas, which is more highly reducing, such as hydrogen (H2) and carbon monoxide (CO), and uses this reducing gas to improve the reduction performance of the lean NOx catalyst while at low temperatures.
Herein, as the reforming reaction of the reforming catalyst, for example, a reaction has been known that produces a gas containing hydrogen and carbon monoxide by the partial oxidation reaction of hydrocarbons such as that shown in the following formula.CnHm+½nO2→nCO+½mH2 
This partial oxidation reaction is an exothermal reaction employing fuel and oxygen, and the reaction progresses spontaneously. As a result, upon the reaction being started, it is possible to continuously produce hydrogen without the supply of heat from outside. In addition, in this kind of partial oxidation reaction, in a case in which fuel and oxygen coexist in a high temperature state, the combustion reaction as shown in the following formula also progresses on the reforming catalyst.CnHm+(n+¼m)O2→nCO2+½mH2O
As the reforming reaction, in addition to the partial oxidation reaction, the steam reforming reaction as shown in the following formula has also been known.CnHm+nH2O→nCO+(n+½m)H2 
This steam reforming reaction is an endothermic reaction employing fuel and steam, and is not a reaction that progresses spontaneously. As a result, the steam reforming reaction is easily controlled relative to the partial oxidation reaction described above. On the other hand, it is necessary to input energy such as of a heat supply from outside.
In Patent Document 3, a device is exemplified that is provided with such a reforming catalyst as described above upstream of a lean NOx catalyst in the exhaust path. In this exhaust purification device in particular, a catalyst capable of performing a plurality of different reforming reactions such as the partial oxidation reaction, steam reforming reaction and water-gas shift reaction is employed as the reforming catalyst. In addition, when NOx desorbs from the lean NOx catalyst, the exhaust air-fuel ratio is enriched, and the main reforming reaction in the reforming catalyst is also switched by switching enrichment control of the exhaust air-fuel ratio according to the temperature of the exhaust. This exhaust purification device has an object of producing the hydrogen necessary in reductive purification of NOx irrespective of the temperature of the exhaust by switching the reforming reaction according to the temperature.    Patent Document 1: Japanese Patent No. 3454336    Patent Document 2: Japanese Patent No. 3627372    Patent Document 3: Japanese Patent No. 3740987